Age and origin of the corundum-bearing rocks of Khitostrov Island, Northern Karelia

The Chupa nappe of the Belomorian Complex contains aluminous silica-undersaturated rocks with corundum, which are characterized by extremely low O-18/O-16 (whole-rock delta O-18 up to -21aEuro degrees). Revealed isotopic anomalies are explained by the influence of meteoric waters that were modified through evaporation-precipitation cycles (Rayleigh distillation) under cold climatic conditions. In order to estimate whether the decrease in delta O-18 occurred prior to metamorphism of the protoliths of the Chupa Sequence or during water percolation in the course of metamorphic or postmetamorphic transformations, we studied oxygen composition in the rocks and minerals and conducted U-Pb dating on single zircons from corundumbearing rocks of Khitostrov. U-Pb zircon dating allowed us to distinguish the following stages: (1) formation of protoliths of the corundum-bearing rocks and protoliths of the Chupa Sequence 2.9-2.8 Ga ago; (2) metamorphism under the amphibolite-facies conditions 2747 +/- 6 Ma ago; (3) metamorphism under the amphibolite and greenschistfacies conditions 1.83 Ga ago. Oxygen isotopic anomalies were presumably formed at the pre-metamorphic stage and were not related to the Svecofennian metasomatism, as was suggested previously (Serebryakov et al., 2007). The possible protoliths for the corundum-bearing rocks could be Archean allochthonous complexes, whose composition, including silica deficiency and high alumina content, was determined by weathering. Meteoric fluid sharply depleted in O-18 could be entrapped and buried with fragments of the weathering crust, and later, together with the protoliths of the Chupa gneisses, was involved into collisional processes and subjected to high-grade metamorphism at 2750-2720 Ma.